This invention relates to improved tantalum powder useful in powder metallurgy and particularly in making electrodes for electrolytic capacitors by powder metallurgy techniques and to electrodes porduced from such powder.
Electrodes and particularly anodes for solid and liquid type electrolytic capacitors have been previously produced from various tantalum powders by various powder metallurgy techniques. In anodes for such capacitors, it is desirable to have as high a specific capacitance (CV/g) as possible along with low DC leakage, low dissipation factor, and high breakdown voltage characteristics.
A previously known capacitor anode and a process and tantalum powder for producing such an anode is disclosed and claimed in Pierret U.S. Pat. Nos. 3,934,179; 3,473,915; and 3,418,106 respectively. As disclosed in these Pierret patents, a capacitor anode may be made by crushing a hydrided tantalum ingot into particles having a size in the range of about 2 to 30 microns, agglomerating the crushed particles, crushing the resulting agglomerated mass to a powder until about 75% by weight will pass through a 325 mesh screen, mixing the resulting powder with a binder, pressing the resulting mixture into a compact, sintering the resulting compact, and anodizing the sintered compact in a phosophric acid solution to form an anodic dielectric film on the sintered compact to thereby produce a capacitor anode.
Binders often of a carbonaceous type such as carbowax, acrawax, and glyptol are usually mixed with the tantalum powder to improve its flow characteristics so that it may be readily dispensed in automatic pellet making or compact forming machines to rapidly produce pellets of uniform weight. Usually binders are also needed to form a compact with adequate green strength to withstand handling and loading prior to sintering. However, use of carbonaceous binders is objectionable because it requires a two step sintering process which involves a first heat treatment to remove the binders followed by a second heat treatment at a higher temperature to sinter the compact. Use of carbonaceous binders also results in the sintered compact retaining residual carbon which tends to adversely affect its electrical properties which are desirable for capacitor anodes.
In producing compacts of powdered tantalum with high speed automatic presses, a portion of the powder is not pelletized or compacted due to intentional overfilling of the dies, and any resulting pellets which are defective are recrushed into powder. Such overfill and recrushed powder is recycled, may ultimately be pressed or compacted several times, and during such recycling tends to be degraded in quality and characteristics desirable for producing capacitor anodes.
An improved tantalum powder and process of making it suitable for producing capacitor anodes by powder metallurgy techniques without utilizing any binders and which is not degraded by recycling is disclosed in Bates et al U.S. Pat. No. 4,017,302. This improved tantalum powder is produced by hydriding high purity tantalum metal ingots, milling the hydrided tantalum metal to a powder of predetermined particle size, subjecting the powder to a two step heat treatment to degas and preagglomerate it, milling and screening the preagglomerated powder to a powder of an intermediate mesh size, subjecting the powder of intermediate mesh size to a high temperature treatment to agglomerate it, milling the agglomerated powder to a powder of finer intermediate mesh size, screening the powder of finer intermediate mesh size to remove a portion of the finer particles thereof, subjecting the removed finer particles to an additional heat treatment to reagglomerate them, milling and screening the removed and reagglomerated particles to finer particles, and blending the removed and milled finer particles with the portions of the coarser particles of the agglomerated powder.
Fry U.S. Pat. No. 4,009,007 discloses that the electrical capacitance of anodes produced from tantalum powder can be increased by the addition to the powder of about 5 to 400 parts per million by weight of elemental phosphorus and that in some instances such addition of phosphorus also improves the flow properties of the powder.